The present invention relates to electrophotographic recording materials, particularly those constituted by selenium, selenium alloys or selenium compounds applied to a conductive carrier and containing arsenic as an additive.
Electrophotographic processes and apparatus for this purpose are widely used in the duplication art. They rely on the property of the photoconductive material to change its electrical resistance when exposed to an activating radiation.
After a photoconductive layer is electrically charged and exposed to an activating radiation, it is possible to produce thereon a latent electrical charge image which corresponds to the optical image which constitutes the activating radiation source. At the exposed points there occurs such an increase in the conductivity of the photoconductive layer that the electrical charge can flow out through the conductive carrier. More precisely, at least part of the stored charge will flow out and in any case more will flow off from exposed points than from the unexposed points. At the same time, at the unexposed points the electrical charge remains substantially unchanged. The resulting charge image can be made visible with a picture powder, a so-called toner, and the resulting toner image can finally be transferred to paper or some other medium, if this should be required.
Organic as well as inorganic substances are used as the electrophotographically active substances. Among them, selenium, selenium alloys and selenium compounds have gained particular significance.
Various demands are placed on the mechanical, optical, electrical and thermal properties of electrophotographically active substances if they are to be employed with success and advantage in practice. No single prior art layer could meet all of these demands. It is known, however, to improve certain properties of electrophotographically active substances through the use of additives.
Thus, the low thermal stability of layers of amorphous selenium, which has a generally undesirable tendency to change to the crystalline state, can be improved by the addition of arsenic. Also, the low hardness of amorphous selenium layers can be improved, according to known processes, by the addition of arsenic in concentrations of up to 1%, with a possible concentration gradient which increases in the direction toward the surface. In a known arrangement the arsenic concentration at the surface is then up to 13%. With such an arrangement the improvement in thermal stability and hardness must be paid for, however, by a reduction in sensitivity.
Homogeneous selenium-arsenic systems are also known in which the atomic mixing ratio reaches 1:1. These systems have a higher sensitivity with higher arsenic concentration than can be observed with pure selenium. The maximum in sensitivity is observed with As.sub.2 Se.sub.3, which corresponds to 38.74% by weight of arsenic.
Such systems with such high proportions of arsenic, however, also have high glass transformation temperatures. For this reason the production of electrophotographic layers exhibiting good adhesion is possible only at very high substrate temperatures in the vicinity of approximately 200.degree.C, which presents technical difficulties.